A number of devices have been designed for the purpose of cleaning out debris having collected at the bottom of a wellbore, as such a deposit can have severely detrimental effect on the flow of production fluids from the well.
U.S. Pat. No. 4,940,092 teaches a well clean out tool connected in series in a drill string and featuring a swab assembly reciprocally supported within a pump barrel at the end of a pump mandrel connected through upper sections of the drill string to a rotational and reciprocal drive source at the surface. The pump barrel is ported near its top end to discharge fluids lifted within it by the swab cups during an upstroke of the tool, the lifting of this fluid reducing pressure within the pump barrel below the swab cup so as to open a check valve disposed in a trapper sub connected below the pump barrel, through which debris-carrying fluid enters the debris-trapping sub to remove the debris from the surrounding bottom region of the wellbore into which the string has been lowered.
The prior art tool may be prone to debris build-up within the pump barrel should any debris carried into the trapper sub not be retained therein, which would require removal of the tool from the wellbore for disassembly and subsequent cleaning of the pump barrel. Furthermore, damage may be caused to the inner surface of the pump barrel wall by contact therewith by the lower end of the swab mandrel under flexing that may be experienced during reciprocation of the pump mandrel and upper string sections.